I can't change the "In game controls" those coloured as red.

Is there a way of changing the controls by means of altering a file or text files?
I'm quite experienced with fps games starting with the CountersStrike years and I've always jumped with TAB and used SPACE for fire to minimise the pistols recoil. It's embeded in my muscle memory. Anyway in the controls section I can not choose TAB for jump. Any way you can help me is much appreciated.
I've tried altering the default.cfg file but that does nothing.

THanks.